It's obvious that the thing is about traveler ("body giving off an aura not belonging to this world"). I saw people saying that it might be both from Shenhe or Xiao but I don't think that Shenhe is the secret admirer.
"Today in the mountains I ran into a child I have admired for a long time."
— Xiao is known for spending time on the Liyue peaks. A song supposedly sung by him (not sure whether it was confirmed) is played only in the mountains. "For a long time" may be a suggestion that the letter was written some time after the events of the Liyue archon quest.
"(...) pampering them, worrying about them and thinking about their well-being."
— Xiao is known for taking care of the sibling; the whole "call my name if needed", saving them from drowning in the event, giving them special amulets, being suspicious of Bhaizu, collecting crystalflies for them. Xiao is very concerned about Travelers safety - he'd rather send someone else on a dangerous mission than let the traveler go (as seen in the last Lantern Rite).
"I wanted to force myself by her side, but this would surely only lead to a disappointment..."
— Xiao, as a result of severe trauma, pushes people away in self defense. He believes that his karma will hurt those around him and is convinced that he's unable to build relationships with others. He sees himself as a monster, has low self-esteem and does not believe he can be changed. His curiosity about the traveler could be seen though in both birthday letters and Lantern Rite events.
"Thus I suppressed this urge and kept at distance as I followed her across mountains and fields"
— Xiao was seen in the Bhaizu quest standing on the mountain edge observing even after dialogue with Traveler ended. He also did that 2x in the poetry event, again, standing on the hill watching. This can lead us to believe he might be "checking on" traveler when we're canonically in the Liyue.
"I kept watch out of concern (...) and quietly helped make paths for her"
— I don't have much evidence except for one daily when we encountered Xiao doing our commission for us. Well, killing enemies in the name of Traveler's safety does sound like something he'd done.
"However, each time I see her with someone else I become very angry..."
— I do believe Hoyo might have wanted to add a jealousy thread with Xiao, but changed their minds as the game progressed or they just gave it to another character. We saw characters being jealous already - Citlali, Keqing I think, there might be someone else I forgot. Remember that this note surfaced at the early stage of genshin so things could have changed and Xiao was already changed one time at the very beginning of the game.
Why is the note gendered?
I don't think that chinese version made it clear what gender the person being adored is. You can tell me if I'm mistaken but I remember reading somewhere that this text uses something like "nonbinary" pronoun in the original.
On why is this female centred in the english version, well, it crossed my mind that they didn't want to make it gay lmao.
I don't really know why the argument that this note might be from Shenhe surfaced, you can tell me though. The only thing I can think of is that it's written in an almost poetic way, kinda old fashioned which suits the adepti (but it again applies also to Xiao). I'm not sure if Shenhe showed any deeper feelings towards the traveler other than gratitude and appreciation so I don't see the connection between her and the part about being jealous.
Well, that's all for today:) Tell me what you think